1. Field of the Invention
The present invention relates to color processing which takes a fluorescent material contained in a medium into consideration.
2. Description of the Related Art
Color matching processing used to match reproduction colors in an image input device, image display device, and image output device associates the reproduction colors among the respective devices in consideration of color reproduction characteristics of a matching target device.
The sequence for obtaining color reproduction characteristics of an image output device will be described below with reference to FIGS. 1 and 2. The image output device forms a printout 22 by printing a color patch 12 on a predetermined medium 11. Then, the color patch 12 is irradiated with light coming from an illuminant (colorimetric illuminant) 13 of a colorimeter, and light reflected by the color patch 12 is received by a light-receiving device 15 via a spectroscope 14, thereby measuring a spectral radiance of the reflected light. By dividing the spectral radiance of the reflected light by that of the colorimetric illuminant 13, a spectral radiance R(λ) of the color patch 12 is calculated (S23). Next, a spectral radiance S(λ) of an illuminant (viewing illuminant) 24 of an output image viewing environment is measured (S25). Tristimulus values XYZ 27 are calculated based on the spectral radiance R(λ), the spectral radiance S(λ) of the viewing illuminant 24, and a color matching function x((λ) y(λ) z(λ) (S26) by:X=k∫R(λ)S(λ) x(λ)dλY=k∫R(λ)S(λ) y(λ)dλZ=k∫R(λ)S(λ) z(λ)dλ  (1)where k=100/∫S(λ) y(λ)dλ, and
an integral range ranges from 380 to 780 nm.
That is, when the image output device prints color patches 12 of a large number of colors on the medium 11, and obtains colorimetric values (for example, tristimulus values XYZ 27) of the respective color patches, thus obtaining the relationship between signal values (for example, RGB values) 21 input to the image output device upon printing the color patches 12 and the colorimetric values. This correspondence relationship represents the color reproduction characteristics of the image output device.
However, when a material that produces fluorescence (for example, a fluorescent material such as a fluorescent whitener) is used as that of a medium (for example, a print sheet) used in image formation, the spectral radiance R(λ) measured by the aforementioned method is often different from that under the viewing illuminant. Note that the fluorescent material emits light in a wavelength range (fluorescence wavelength range) different from an excitation wavelength range contained in illumination light. In general, a fluorescence wavelength is longer than an excitation wavelength.
Graphs of FIGS. 3A and 3B show measured values of radiated light from a color patch when the color patch containing a fluorescent material is irradiated with monochromatic light. FIG. 3A shows the intensities of radiated light when the color patch is irradiated with monochromatic light of 350 nm. Radiated light 1101 is reflected light of the irradiated monochromatic light, and radiated light 1102 is fluorescence excited by the irradiated monochromatic light. On the other hand, FIG. 3B shows the intensities of irradiated light when the color patch is irradiated with monochromatic light of 440 nm. Radiated light 1103 is reflected light of the irradiated monochromatic light.
As shown in FIG. 3A, when the color patch contains a fluorescent material, and is irradiated with light of an excitation wavelength, the fluorescence 1102 having a wavelength different from that of the irradiated light is observed in addition to the reflected light 1101 of the wavelength of the irradiated light. On the other hand, as shown in FIG. 3B, when the color patch is irradiated with light of a wavelength different from the excitation wavelength, the reflected light 1103 of the wavelength of the irradiated light is observed. For this reason, under an illuminant including, for example, a 350-nm component and 450-nm component, light of 440 nm observed as the radiated light of the color patch is a sum of the fluorescence 1102 and reflected light 1103. Of course, since a general illuminant has many wavelength components, a sum total of the reflected light of 440 nm and fluorescence components of 440 nm for their respective wavelengths becomes a radiated light of 440 nm observed from the color patch under that illuminant.
The spectral radiance R(λ) of a sample containing a fluorescent material, an excitation wavelength which falls within an ultraviolet range, will be described below with reference to FIGS. 4A to 4C. When radiated light of the sample containing the fluorescent material is measured using the measurement system shown in FIG. 1, the fluorescent material reacts with light in an ultraviolet range 41 in FIG. 4A contained in the radiated light, thus emanating light in a fluorescence wavelength range 42 in FIG. 4B. That is, a colorimeter receives, from the sample, radiated light added with fluorescence which depends on a light energy in the ultraviolet range (excitation wavelength range) 41 of the colorimetric illuminant 13. As a result, the spectral radiance R(λ) also depends on the light energy of the ultraviolet range 41 of the colorimetric illuminant 13 (FIG. 4C). When the colorimetric illuminant 13 is the same as the viewing illuminant 24, since fluorescence corresponding to the light energy in the excitation wavelength range of the viewing illuminant 24 is obtained upon measurement, correct colorimetric values are calculated. On the other hand, when the colorimetric illuminant 13 is different from the viewing illuminant 24, since fluorescence, which does not correspond to the light energy in the excitation wavelength range of the viewing illuminant 24, is obtained upon measurement, correct tristimulus values cannot be calculated.
As a method of obtaining tristimulus values from a sample printed on a medium containing a fluorescent material, a method using a bi-spectral radiance factor is available. The bi-spectral radiance factor is a bivariable function F(μ, λ) that represents a spectral radiance of a sample with respect to incident light of a wavelength μ, and can express a fluorescence amount radiated in a wavelength range different from that of irradiated light. Using the bi-spectral radiance factor, CIEXYZ values that consider fluorescence can be calculated using:X=k∫λ{∫μF(μ,λ)S(μ)dμ· x(λ)}dλY=k∫λ{∫μF(μ,λ)S(μ)dμ· y(λ)}dλZ=k∫λ{∫μF(μ,λ)S(μ)dμ· z(λ)}dλ  (2)where k=100/∫λS(λ)y(λ)dλ,
an integral range of ∫λ ranges from 380 to 780 nm, and
an integral range of ∫μ ranges from 300 to 780 nm.
However, measurement of the bi-spectral radiance factor requires a measuring device such as a bi-spectral fluorescence colorimeter which includes spectroscopes on both the illumination and light-receiving sides. In addition, the bi-spectral radiance factors for incident light have to be measured per sample in increments of 10 nm within the range from 300 nm to 780 nm, and the measurement requires an immense amount of time. In other words, it is not practical to prepare a large number of color charts obtained by printing a large number of color patches on a large number of media, and to measure bi-spectral radiance factors for respective color patches.
In case of a sample printed on a medium containing a fluorescent material, only excitation light according to a spectral transmittance of a color material reaches the medium on a portion where the color material covers a surface of the medium. Therefore, fluorescence radiated from the medium depends not only on the excitation light amount and the fluorescence characteristics of the medium but also on the spectral reflectance of the color material and a factor at which the color material covers the surface of the medium (to be referred to as a covering factor hereinafter). Japanese Patent Laid-Open No. 2006-292510 (patent literature 1) describes an invention that estimates bi-spectral radiance factors of a sample based on bi-spectral radiance factors of a medium, a spectral transmittance of a color material, and a covering factor of the color material.
According to the invention of patent literature 1, as an estimation reference of the covering factor, a reference printed surface on which color materials alone and combinations of a plurality of color materials are printed at a covering ratio of 100%, and the spectral radiances of the reference printed surface are measured. Next, the spectral radiances of samples are measured, and the covering ratios of the color materials on the samples are estimated based on the spectral radiances of the reference printed surface. Then, the bi-spectral radiance factors of the samples are estimated from the estimated covering ratios, bi-spectral radiance factors of a medium, which are measured in advance, and spectral transmittances of the respective color materials.
The invention of patent literature 1 prepares for the spectral radiances of the reference printed surface, the bi-spectral radiance factors of the medium, and the spectral transmittances of the respective color materials in advance, so as to estimate the bi-spectral radiance factors of the samples. Furthermore, in order to estimate the covering ratios, total spectral radiances of the samples have to be measured under a measurement illuminant. Therefore, according to the invention of patent literature 1, measurements can be simplified compared to a case in which bi-spectral radiance factors of a large number of color patches are measured. However, various kinds of information have to be prepared for estimation, and the invention of patent literature 1 is not practical.